Ultrasound is the most widely used diagnostic modality in the world (with the possible exception of traditional X-ray). There remains a need for improved transducer performance. In this context, the transducer active material, usually but not always piezoelectric, is supported on a substrate that dissipates ultrasound energy that is propagated into it. (Only ultrasound energy emanating from the front of the transducer is desirable since it propagates into the tissue.) The backscattered ultrasound signals are received by the same transducer array, converted to electrical signals and processed in attached beamformer electronic hardware.
Conventional transducer backing blocks comprise a blend of various subsets of: polymer (usually, but not always an epoxy), fibers (or short fiber segments), particulate fillers (usually metal powders or metal oxides—such as Al2O3 or SiO2), bubbles (such as free gas, glass or polymer microballoons) and sometimes “rubberizing” agents such as modifiers (plasticizers) appropriate for the polymer used.
Frequently, the electrical “signal” connections to each of array elements is made on the back surface of the transducer—since there is more space there than on the front surface where a very short path between the active transducer material and the tissue is required. The electrical connections are sometimes made using IC die wirebonding techniques (thermal or ultrasonically induced gold wire bonds) or using a thin polyimide (or similar) flexible circuit with plated (e.g. gold) copper traces that align with the transducer elements. In the context of highly complex designs—such as a 2D array—it becomes practically impossible to design a flexible circuit to provide the necessary signal line “fan-out”. Wirebonding techniques become highly problematic.
Commercially available Printed Circuit Board (PCB) materials (used as the substrate in a transducer, and containing the copper traces) can provide a low cost method of fanning out the element traces from the array to connectors (or to “front end” ICs such as protection circuits, pre-amplifiers, digitizers, etc,), however, the materials within the PCBs have insufficient attenuation and parasitic echoes are received from within the PCB and from the reflective surface at the back of the PCB. Typically, the PCBs are filled with woven glass fiber that is potted in a polymer—possibly a polyester polymer—though the exact composition of the polymer is immaterial to the current discussion and is rarely described in detail. Since the primary application of commercial PCB materials is as substrates for Printed Circuit Assemblies (PCAs) that are frequently subject to solder reflow oven processes, the polymer is chosen to have a glass transition temperature that is sufficiently high (˜200° C.) to avoid excess geometric distortion during solder reflow. (“Reflow” is the process by which previously placed solder “tinned” components are permanently attached by way of “reflow” melting the adjacent solder surfaces on both the component “legs” and the PCB solder tinned “pads”).
Therefore there exists a need in the art to provide backing block materials which can improve transducer performance through increased attenuation of parasitic echoes generated within the devices during their operation.